为什么该程序在我的系统上终止而不在操场上终止？

Question

Consider this program:

package main

import "fmt"
import "time"
import "runtime"

func main() {

    x := 0

    go func() {
        time.Sleep(500 * time.Millisecond)
        x = 1
    }()

    for x == 0 {
        runtime.Gosched()
    }

    fmt.Println("it works!")
}

Why does it terminate locally but not on Playground? Does the termination of my program rely on undefined behavior?

Answer 1

That code doesn't offer much guarantees. It's relying almost entirely on implementation details around undefined behavior.

In most multi-threaded systems, there's no guarantee that a change in one thread with no barrier will be seen in another. You've got a goroutine which could be running on another processor altogether writing a value to a variable nobody's ever guaranteed to read.

The for x == 0 { could quite easily be rewritten to for { since there's never a guarantee any changes to that variable might be visible.

The race detector will also probably report this issue. You should really not expect this to work. If you want a sync.WaitGroup you should just use one as it properly coordinates across threads.